A car body of an automobile includes various structural members (e.g., a front side member, a rear side member, a center pillar reinforcement, and the like). For the structural members, press-formed products are heavily used, and the structural members are formed from a single press-formed product or formed by joining a plurality of press-formed products. The press-formed product used in the structural member has an open cross-section including a bent portion, and the cross-sectional shape thereof is hat-shaped or groove-shaped. Hereafter, a hat-shaped or groove-shaped cross-sectional shapes are also collectively referred to as hat-shaped. In a hat-shaped press-formed product, the bent portion of the cross section forms a ridge portion in appearance. Such a press-formed product is produced by using a planar steel plate as a starting material and subjecting the blank steel plate to press working.
In recent years, the fuel saving of automobiles has been promoted as one of the countermeasures against global warming problem. Thus, automobile bodies have a demand of reducing the weight thereof and securing a collision safety performance at the same time. To meet this demand, the strength of structural members are appropriately set in conformity with mounting location of the structural members in a car body. Furthermore, even in one structural member, the optimization of strength is made for each region.
In order to make the optimization of strength in a structural member, as a blank steel plate, a tailored welded blank (hereafter, referred to as a TWB), a tailored rolled blank (hereafter, referred to as a TRB), or the like has been employed. A TWB is a blank made by joining a plurality of kinds of steel plates by laser welding or the like, and has strength differences and thickness differences. A TRB is a blank made by rolling while the axis intervals between rolling mill rolls are adjusted in producing a steel plate, and has thickness differences and thereby substantially strength differences. A structural member produced from a TWB or TRB is enhanced in strength at appropriate spots and reduced in weight.
In addition, in a structural member using a hat-shaped press-formed product, when the thickness of a ridge portion, namely the thickness of a bent portion is larger than those of the other portions, the strength of the structural member is enhanced, and an impact energy absorbing performance or the like is enhanced. For such an advantage, a ridge portion of a structural member may be subjected to buildup welding.
A technique in which a TWB or a TRB as a blank steel plate is employed, or a technique in which a press-formed product is subjected to buildup welding, is effective to reduce the weight of a structural member and secure a collision safety performance. However, the technique in which a TWB is employed as a blank steel plate, or the technique in which a press-formed product is subjected to buildup welding compels the addition of a large-scale welding facility, requiring the augmentation of a complicated welding process. The technique in which a TRB is employed as a blank steel plate compels the addition of a large-scale rolling facility. Therefore, a significant rise in manufacturing costs is inevitable in both prior arts.
In contrast to the above prior arts that require a large-scale welding facility or rolling facility, press working with a pressing facility is excellent in multiplicity of use. For this reason, if the thickness of a structural member can be partially increased by press working to thereby partially enhance the strength of the structural member, the manufacturing costs of a structural member could be suppressed. Techniques in which the thickness of structural member is partially increased by press working will be described in the following literature.
Japanese Patent Application Publication No. 2010-120061 (Patent Literature 1) discloses a press-formed product and a production method therefor. The press-formed product described in Patent Literature 1 has an open cross-section including bent portions, and the cross-sectional shape thereof is hat-shaped. The press-formed product includes a pair of vertical wall portions and a top panel portion and curves in a gentle L shape in plan view. In the press-formed product, ridge portions that couple the vertical wall portions with the top panel portion correspond to bent portions of the cross sections. Furthermore, the thickness of the vertical wall portions on the inside of the curve and a part of the top panel portion adjacent to the vertical wall portion on the inside of the curve (including the ridge portions) is increased as compared to the portions other than thereof.
The press-formed product described in Patent Literature 1 is produced through the following process. A blank steel plate having a constant thickness is subjected to bending forming by press working and formed into a hat-shaped preparatory formed body. The preparatory formed body has a substantially constant thickness across its entirety, and the inside of the curve of the top panel portion is broadened inwardly in the curve together with the vertical wall portion, as compared with a press-formed product, which is an end product. Then, by press working using the other press die, the vertical wall portion on the inside of the curve of the top panel portion is pushed outwardly in the curve. At that point, a part of the top panel portion adjacent to the vertical wall portion on the inside of the curve is compressed to expand, and the thickness thereof increases. The resulting press-formed product has an increased thickness in a ridge portion on the inside of the curve.
Japanese Patent Application Publication No. 2008-296252 (Patent Literature 2) discloses a hat-shaped press-formed product and a production method therefor. The press-formed product described in Patent Literature 2 includes a pair of vertical wall portions and a top panel portion. Furthermore, in a specific range in a longitudinal direction, the thicknesses of the vertical wall portions and the top panel portion (including a ridge portion) are increased.
The press-formed product described in Patent Literature 2 is produced through the following process. A blank steel plate having a constant thickness is subjected to bending forming by press working and formed into a hat-shaped preparatory formed body. The preparatory formed body has a substantially constant thickness across its entirety, and the vertical wall portions extend in the specific range in the longitudinal direction, as compared with a press-formed product, which is an end product. Then, by press working using the other press dies, the top panel portion is pushed. At that point, the vertical wall portion is compressed to expand while being bent, and at the same time the top panel portion is crushed while being bent and expands, and the thicknesses thereof increase. The resulting press-formed product has an increased thickness in the vertical wall portions and the top panel portion (including a ridge portion) in the specific range in the longitudinal direction.
Japanese Patent Application Publication No. 2007-14978 (Patent Literature 3) discloses a hat-shaped press-formed product and a production method therefor. The press-formed product described in Patent Literature 3 includes a pair of vertical wall portions and a top panel portion. Furthermore, only in ridge portions that couple the vertical wall portions with the top panel portion, the thickness thereof is increased.
The press-formed product described in Patent Literature 3 is produced through the following process. A steel plate having a constant thickness is sandwiched between a pair of forging dies vertically disposed. On the opposing faces of these forging dies, recessed portions are formed at positions corresponding to ridge portions of the press-formed product. Furthermore, in the forging dies, heaters are embedded in the vicinities of the recessed portions. The steel plate is locally heated by the heaters, with the steel plate sandwiched between the forging dies. After the heating, upset dies disposed on the right and left side of the above forging dies are caused to operate to compress the steel plate in a direction perpendicular to the thickness direction. At that point, the steel plate buckles in the vicinities of the recessed portions of the forging dies, and flows into the recessed portions of the forging dies. This yields a planar blank steel plate having a partially increased thickness in areas corresponding to the recessed portions of the forging dies.
Subsequently, the planar blank steel plate having the partially increased thickness (hereafter, also referred to as a “partially thickened blank”) is subjected to bending forming by press working using the other press dies. At that point, the press working is performed such that the thickened areas of the partially thickened blank form ridge portions (bent portions in a cross section) of the press-formed product. The resulting press-formed product has a thickness that increased only in the ridge portions.